The present invention relates to a cylindrical side reflector for a nuclear reactor having annuli superposed on each other. A plurality of graphite blocks form the annuli with the graphite blocks extending radially through the entire thickness of the reflector wall. The invention is particularly related to gas-cooled high temperature reactors having a bed of spherical fuel elements.
In all nuclear reactors having graphite side reflectors radiation damage occurs in the graphite after an extended period of operation or with a high power density in the core of the reactor even after a shorter period of time. The radiation damage potentially leads to the crumbling of the parts of the reflector facing the reactor core and possibly to the shutdown of the operation of the reactor. The primary cause of this phenomena is that the flux of rapid neutrons coupled with the power density causes, on the basis of the Wigner effect, dimensional changes in the graphite. Initially, there is shrinking, followed after a certain period of time by expansion with the latter continuing beyond the initial state. The stresses generated in the process may lead to cracking of the graphite. However, side reflectors are expected to have lives under full loads of up to 35 years. The radiation exposure of side reflectors is particularly high in nuclear reactors with spherical fuel elements which must pass through the reactor core only once.
In such a nuclear reactor the spherical fuel elements have attained their final state of depletion desired after a single passage. In the process, a configuration of power density distribution declining from top to bottom is established, related to a high dose of rapid neutrons in the upper third of the reactor core.
Nuclear reactors with spherical fuel elements (pebble bed reactors) have been developed in the prior art wherein the premature shutdown of the reactor operation is avoided by a particular configuration of the side reflector. Thus, a nuclear reactor is known from West German Pat. No. 10 34 784 having a reflector jacket consisting of a bed of uniformly shaped neutron reflecting bodies. The bodies essentially have the configuration of the fuel elements. Similarly, in Austrian patent application No. 9582/63 a nuclear reactor is described with a side reflector consisting of a bed of spherical graphite elements. These elements flow continuously through the reactor core and are constantly replaced by new graphite spheres, so that the exposed part of the side reflector is constantly replaced.
It is known from West German Offenlegungsschrift No. 23 52 691 to design the boundry zone immediately adjacent to the side reflector of the bed of fuel element in a special manner for the protection of the solid side reflector against an excessive dose of fast neutrons, by charging the boundary zone with fuel elements having a lower content of fissionable material than the inner zone of the bed. It has also been proposed to make the boundary zone between the solid side reflector and the bed of fuel elements very narrow and to charge it with pure graphite pebbles.
Another measure to protect the most intensively exposed part of the side reflector is described in West German Offenlegungsschrift No. 23 47 817. This measure consists of providing within the top reflector and the upper part of the solid side reflector materials which absorb neutrons or reduce the velocity of the neutrons. This results in a significant reduction of the flux of fast neutrons in the areas endangered by radiation of the reflector. The materials may be in the form of rods and may be located in suitable cavities. In this known reactor the fuel elements pass only once through the core of the reactor.
In a further nuclear reactor employing a single passage of the spherical fuel elements described in West German Offenlegungsschrift No. 26 12 178, damage to the side reflector is prevented by a specially designed shutdown and control system. This comprises among others a plurality of absorber rods inserted into the side reflector and movable therein. The absorber rods are always located (to a depth of 70%) in the side reflector and reduce the neutron flux in the reflector.
There are also known processes for the replacement of parts of side reflectors composed of square stone-like blocks. Thus, in West German Offenlegungsschrift No. 25 09 025, the operating mode of a manipulator is explained. Individual reflector blocks are removed from the reactor core with the aid of the manipulator. Even though the premature shutdown of the nuclear reactor due to unpredictable damage to the graphite inserts may be prevented in this manner, the replacement or repair, respectively of the damaged parts of the reflector is very time consuming and costly.
In order to avoid these disadvantages, West German Offenlegungsschrift No. 26 43 275 proposes a graphite side reflector built of blocks, which would make the replacement or exchange of reflector blocks unnecessary over the entire lifetime of the nuclear reactor. This is achieved by providing the individual blocks, which extend continuously in the radial direction over the entire thickness of the reflector wall, on their internal front surface with recesses consisting of a series of joints. This measure is based on the concept of simulating the existence of small block dimensions within the area of blocks exposed to particularly high stresses due to neutron irradiation and heat effects. In blocks with significantly smaller internal front surfaces no appreciable stress cracking occurs (however, smaller blocks are more expensive to produce and difficult to install). The disadvantage of this known side reflector rests in the fact that the internal surface of the blocks must be machined, which leads to new problems.